The general process for making a porous polymer by mixing the polymer with a second polymer or a second type of material which can be leached from the mixture is known in the art as exemplified by U.S. Pat. No. 2,504,208; which discloses the general concept of making a porous polymer by mixing soluble and insoluble polymers and then leaching the soluble polymer from the mixture to form a porous product (Col. 1, lines 28-40).
U.S. Pat. No. 3,062,760 discloses a process for making a porous polymer comprising mixing a polymer with a pore forming material, extruding the mixture into a shape and removing the pore forming material by leaching. Of particular interest is the disclosure of the use of calcium carbonate as a suitable pore forming agent and the teaching that such a pore forming agent can be removed from the polymerized resin by a leaching agent such as an acid (Col. 2, lines 5-23).
U.S Pat. No. 3,310,505 discloses a method for making a porous polymer wherein a mixture of polymers is subjected to leaching to remove one of the polymers from the mixture.
U.S. Pat. Nos. 3,682,848, 3,763,055, 4,705,809, 4,096,099, and 4,237,083 disclose porous polymers for use as filters and microporous membranes.
U.S. Pat. No. 4,351,860 discloses a polyaryl ether sulfone semipermeable membrane and a process for producing the same. A polyaryl ether sulfone is dissolved in a mixed solution of electrolyte and solvents to form a dope of the polyaryl ether sulfone which is extruded to obtain a film or fiber. Both sides of the fiber or film are contacted with a liquid miscible with the mixed solvent but incapable of dissolving the polyaryl ether sulfone so as to remove substantially all of the mixed solvent from the fiber or film to obtain a porous membrane.
U.S. Pat. No. 4,163,725 discloses a method for the preparation of a semipermeable membrane involving polysulfones. Porous membranes of polysulfone are prepared by dissolving polysulfone in a solvent. The polymer solution is cast on a substrate and the solvent is evaporated for a period of time. The substrate is then dipped in a non-solvent.
U.S. Pat. No. 4,612,119 discloses a process for preparing a hollow fiber filter medium by dissolving a polysulfone in a solvent to form a solvent solution, extruding the solvent solution and bringing the extrudate into contact with an outside coagulating liquid. The transition temperature of the solution is within the range of 30.degree. to 150.degree. C., which corresponds to the range of 86.degree. F to 302.degree. F.
It is an object of the present invention to provide an improved material for water filtration, and for an improved process for obtaining said material.